As is well known, in active pixel image sensors an area of the pixel acts as a photodiode, with photon-generated current being integrated on the self-capacitance of the photodiode. This charge is essentially an analog representation of light received at that pixel during the exposure period. When a digital signal is desired, it is necessary to provide A-D conversion.
Most active pixels use one or more A-D converters located off the image plane. This maximizes the light-converting properties of the image plane, but at the expense of requiring a relatively complex switching or multiplexing arrangement to transfer pixel signal values to the A-D converters.
Layouts have been proposed in which each pixel has its own A-D converter; see for example U.S. Pat. Nos. 5,461,425 and 5,801,657 to Fowler et al., U.S. Pat. No. 6,271,785 to Martin, and IEEE Journal Solid State Physics, December 2001, Vol. 36, No. 12, p. 2049 (Kleinfelder et al). However, these layouts have a disadvantage in that the additional circuitry in each pixel severely reduces the ability of the pixel to collect photon-generated electrons, and thus severely reduces sensitivity.